1. Field of the Invention
The present invention relates to an improved method for the gas soft-nitriding treatment of iron parts by employing a carrier gas containing a major proportion of nitrogen gas.
2. Description of Relevant Art
In general, iron members to be used as parts for machine construction are required to have superior fatigue and wear resistance qualities. In order to satisfy such requirements, there are known methods in which iron parts are nitrided to form a hard layer composed of a nitride compound in the surface portion of the iron parts, imparting the required characteristics to the parts. Typical of such methods are: a known gas nitriding method; a salt bath nitriding method; and the like. However, such methods have a number of attendant disadvantages and shortcomings.
For example, the known gas nitriding method requires an undesirably prolonged treating time, and presents a problem with regard to fatigue resistance because the parts treated by the method become relatively fragile, though showing high hardness due to formation of a deep hardened layer. Accordingly, such method is unfavorable if the parts require an excellent fatigue resistance quality with a degree of hardness.
The salt bath nitriding method is advantageous in that the treatment is feasible at low temperature with a shorter treating time and that the resultant parts are relatively high in hardness and fatigue resistance, and are thus favorably usable as iron machine construction parts which should have high fatigue resistance. In contrast to the above advantages, however, the salt bath nitriding method requires use of cyanides or cyanates which are highly toxic, presenting problems such as environmental pollution, and inconvenience and danger in handling, etc.
In recent years, there have been developed, proposed, and put into practice gas soft-nitriding methods in which iron parts are nitempered under relatively safe conditions using a gaseous medium containing nitrogen gas to improve the fatigue strength and wear resistance of the parts. These gas soft-nitriding methods ensure similar treating results as the salt bath nitriding method, with the advantage that the problem of environmental pollution is substantially alleviated.
Such gas soft-nitriding method entails formation in the surface portion of iron parts a layer of crystalline structure called an .epsilon. phase composed of iron, nitrogen and carbon (Fe-N-C). Known carrier gases useful in the gas soft-nitriding treatment have the following compositions.
(1) Combination of ammonia gas and an endothermic gas (RX gas)
NH.sub.3 gas: 50%
RX gas: 50%
(NH.sub.3 50%, CO 12.5%, H.sub.2 15.6%, CO.sub.2 0.13%, and the remainder N.sub.2, such percentages, as well as those presented hereinbelow, being by volume).
(2) Combination of ammonia gas and methanol-cracked gas
NH.sub.3 gas: 50%
Methanol-cracked gas: 50%
(NH.sub.3 50%, CO 16.7%, H.sub.2 3.3%, CO.sub.2 0.2%, and the remainder N.sub.2).
(3) Combination of ammonia gas and an exothermic gas (NX gas)
NH.sub.3 gas: 20%
NX gas: 80%
(NH.sub.3 20%, CO 1.4%, H.sub.2 0.7%, CO.sub.2 0.04%, and the remainder N.sub.2).
Known gas soft-nitriding methods using the above set forth carrier gases have the following disadvantages. With the methods using the carrier gases (1) and (2), ammonia gas which is relatively expensive is used in large quantities (40%-60% by volume), increasing the production cost and the cost of the treated parts. Further, the CO gas is employed in excessive amounts, so that the content of carbon in the resulting compound layer becomes high and thus the hardness of the layer becomes too high due to the large content of carbon. Consequently, the layer tends to be hard and fragile. Particularly, with long parts which are required to be corrected in strain after the treatment, there is a strong likelihood of producing cracks. In addition, the treated parts are deposited with carbon on the surface thereof, and must be washed in an additional step after the treatment.
In the case of using the carrier gas (3) above, the CO content is so small that formation of the .epsilon. phase is not ensured, resulting in poor fatigue strength.
An explanation as to why such problems are encountered in such known methods is set forth hereinbelow.
The fundamental principle on which the gas soft-nitriding method is based is the formation of a Fe-N-C layer of .epsilon. phase in the surface portion of iron parts by diffusing the nitrogen [N] and carbon [C] from the surface of the parts into the interior thereof according to the following reactions: EQU NH.sub.3 .revreaction.[N]+3/2H.sub.2 (a) EQU 2CO.revreaction.[C]+CO.sub.2 (b).
It is important to note that the nitriding treatment should be conducted by use of a gas composition which is capable of producing optimum potentials of [N] and [C].
On the basis of the reaction formula (a), the potential of [N] can be expressed as follows: EQU a[N]=P.sub.NH.sbsb.3 /[P.sub.H.sbsb.2 ].sup.3/2 .multidot.K.sub.N(a.sub.1).
The potential of [C] on the basis of the reaction formula (b) can be expressed as follows: EQU a[C]=[P.sub.CO ].sup.2 /P.sub.CO.sbsb.2 .multidot.K.sub.C (b.sub.1).
The activities of nitrogen [N] and carbon [C] are determined based on the equations (a.sub.1) and (b.sub.1). In the above equations, K.sub.N and K.sub.C represent equilibrium constants of the reaction formulae (a) and (b), respectively.
In a gas soft-nitriding method using 40%-60% of ammonia gas and correspondingly 60%-40% of an endothermic gas (RX gas), the nitriding treatment is not possible unless the ammonia gas is used in a large amount in order to keep the activity of [N] and the value of [N] constant because of a high content of H.sub.2 in the endothermic gas. Use of expensive ammonia gas in such large amount results in an increase of production costs, and a corresponding increase of the cost of treated parts.
In a gas soft-nitriding method using 10%-30% of ammonia gas and correspondingly 90%-70% of an exothermic gas (NX gas), the CO content in the mixed gas is very small, so that it is almost impossible to keep the activity of [C] in the above equation (b.sub.1) at a satisfactory level. Thus, this method is disadvantageous as compared with the method using ammonia gas and the endothermic gas in that the treated parts are more susceptible to fatigue when undergoing stress concentration during application as machine construction components.
The present invention effectively solves the foregoing problems inherently found in conventional gas soft-nitriding methods, i.e., cost problems due to a high consumption of expensive ammonia gas, and problems of a poor resistance to fatigue of the treated parts and deposition of carbon on the treated surface of the parts. As described in the known methods (1)-(3) using large quantities of ammonia gas, it has been found that the content of CO gas has a great influence on the hardness, resistance to fatigue, and wear of parts treated by the gas soft-nitriding method.